Class AB vs. Class A

[SteveG]

Ok, this may be a no-brainer, but when you are building a push-pull tube amp, is there really any benefit of going full class A instead of Class AB with a fair amount of bias? Dosen't the transformer keep the balance so well that the crossover distortion in a class AB would be practically eliminated with a little feedback? The AB is running A up to a certain point anyway, right? Or, is it the A's ability to go without feedback that creates a better sound? This is something I have been wondering about for a while now. Any experiences or thoughts to shed some light on this would be appreciated.
Steve

 

[TG]

Hello SteveG,

My opinion is: "1 good Watt is better than 1000 bad ones"

Now from my experience...
I don't like NFB. This came after about 10 amplifiers I made.
Please don't ask me to explain this from scientific point of view.
It just sounds "wrong" to me...
Class AB, no NFB amplifiers are awful. It's much easier and cheaper to built good solid state amp - they can sound just the same.
By the way, you stress both power and output transformers in AB much more than in A. Maybe this affects sound too...

Good Luck!!!

 

[SteveG]

TG,
Aha!- I forgot about uneven current draw vs. relatively constant current draw on the power transformer, and the fact that the currents in the output transformer cancel with class A.
I don't care for NFB either, but like you said- a class AB sounds horrible without it. It's definitely needed if you are straying out of class A.
The reason I was asking about this- I have been thinking about building a class A p-p with 3 pairs of 6L6's and a plitron pat4008 running class A. I want to make sure that it is worth it to go full A over AB. I guess I really don't have a choice, as the transformer will be past it's limit if I don't go class A. I was thinking of using a differential pair input (possibly cascode) with 6SN7 drivers direct coupled to the input stage. Any suggestions on input tubes?
Steve

 

[PassFan]

While not very opinionated about NFB either way, I do feel it has it's place. When used to compensate for a bad design, no. When used to make a great design even better, yes. At least with the latter you have a choice, and if attention is paid to whether the amp is on or off, a switch can be set up to flip back and forth. I have a SE 6L6 amp with a 6SL7 driver and while the quality is excellent I recently built an older push-pull design from Magnavox that surprisingly sounded a lot smoother and more detailed and lifelike. The circuit can be found at Gabevees site and if your interseted, I can help with tranny selection as it is not on the schematic. Gabe has actually redesigned this circuit and was offering a kit based on that but he is ill and has temporarily shut down kit sales until he is better. At the very least the driver in this amp is the 6EU7 and sounds excellent. It might work well for you if you are going to have more than two stages. While similar to the 12AX7 it sounds different and actually behaves different as a direct plug in replacement. It certainly makes the difference in the Magnavox circuit. Good Luck

 

[TG]

SteveG,
IMHO Class A is the only choice if you gonna use toroidal OPTs.
Toroids saturate easily with even small current unsymmetry, so it will be distortion generator and not the amplifier

6SN7 is a great tube for line stages. Highly recommended to start with. Maybe you can substitute it for even better sound, but it will be pretty hard and amp dependent. As for cascodes - 6L6 is not heavy load for 6SN7, so usual common catode stage will also do.
Some people like the sound of cascode, some don't. Try both then decide...

Good Luck!!!

 

[SteveG]

Thanks for the suggestions TB,
When I build it, I will be going class A- it only makes sense, really. I will have to use a follower to drive the output stage though, because I will be using 3 pairs in parallel push pull. I will probably only use a cascode if I need more voltage gain, but I'm not sure yet. I would like to keep the amp fully symmetric all the way through.
SteveG

 

[SteveG]

sorry, that was TG, not TB. I apologize for the mix up!

 

[dice45]

TG,

thanxalot for the input !

 

[Thatch_Ear]

I built a SE 6AQ5 amp that sounded ok but not great. I added NFB and it really improved to I just don't want to listen to it from I can't take it. An idea I have seen incorperated into a SE was a stepped attenuator in the NFB circuit. Listening showed some difference according to music. Vocals seemed a bit nicer with feedback. Instrumentals were better without any. It does not have to be an either or situation. Everything has its place, even in SE. I don't have it in my amp, but one day when I can spend a bit on the attenuators and resisters I might.

 

[Ryder]

NFB

I'm just learning in this area. Having gone through Morgan Jones book and Bruce Rozenblits book on valve amps I note both think NFB has its place and is useful. Also, both raise their brows a bit over the strong feelings on SETS and anti NFB that is so common these days. I've only started on my first kits and my listening is limited compared to many around here so i'm not trying to start any arguments. Must say I found it interesting when I read the books because I had really settled into the SET no NFB camp prior to reading their books.

Only time and listening will tell.

Cheers,
Ryder

 

[SteveG]

I will agree that feedback has it's place... as someone already said, it's a good tool to put a GOOD design over the edge. But, at the same time, I think the appeal of tubes (besides their distortion characteristics) is the fact that you can get the job done with very few gain stages, and while the distortion is relatively high compared with solid state designs, it is of low order, and is not irritating or harsh. NFB, it seems to me, is a subtractive process. I say that you should build gain as linearly as possible with as few components in the signal path as possible, and then if you have some left over it may be alright to try some NFB. It could put it over the edge. Looking at the numbers, is it really worth it to put a design that is making .5% distortion at typical average listening levels into the .0X% range? I'm not convinced that the cure isn't more harmful than the disease, in a case like this. Distortion on the peaks seems fairly natural to me- our ears produce it. As long as it is clean on average, and the order is low and the amount fairly low on peaks, I don't see the problem with it. Also, if we have efficient speakers, it seems fairly simple to keep the amp in a low distortion area at sane volume levels. Just my .02